This invention relates to an angle of attack indicator and, more particularly, the invention is concerned with providing a diaphragm type instrument for use on aircraft to give an indication of the angle of attack based on movement of the point of maximum pressure down around the leading edge of the wing, or other aerodynamic shape, as the angle of attack is increased and the pressure decreases as one moves away from the point of maximum pressure.
The "angle of attack" may be defined as the term given to the angle of air flow relative to the chord line of a wing. This value is critical during most phases of flight especially including takeoff, climb, cruise, glide, approach, missed approach and landing. In takeoff the normal procedure is to head the aircraft down the runway at full throttle and accelerate the aircraft to takeoff airspeed and then apply a slight amount of back pressure to the control column. The airspeed must then increase to a point of providing sufficient lift to support the aircraft in flight. Since we are depending on airspeed alone, the lift-off point is rather uncertain and can be unnecessarily delayed by too little or too much back pressure on the control column. Too little back pressure will cause the aircraft to attain higher than normal speed before sufficient lift is obtained. This can produce a time delay and result in the use of additional runway. Too much back pressure to the control column will cause a decrease in the forward velocity of the aircraft because of the increase in drag, resulting in a loss of lift and an aborted takeoff.
Since the wing lift is related to its angle of attack, an indication of this angle to the pilot can improve his takeoff procedure and make his control function positive. The takeoff procedure utilizing the angle of attack information would be to proceed down the runway while accelerating to the takeoff airspeed recommended for the particular aircraft and then apply a steadily increasing back pressure to the control column while referring to the angle of attack indicator. Utilizing this procedure, the aircraft will become airborne when its wing angle of attack is the proper value for sustaining lift. By following the above procedure the aircraft will become airborne in the least distance with a positive reference to the actual wing lift.
During final approach and landing, it is important to maintain the aircraft at the minimum maneuvering airspeed which is normally 30 per cent above stall airspeed. The actual airspeed is a compromise at the discretion of the pilot to compensate for varying conditions of aircraft load, weather and wind velocity and windshear. However, because of the aircraft's inertia as well as the natural lag in response of the airspeed indicator, it is difficult for the pilot to keep the airspeed constant and, consequently, keep the glide angle constant. Thus, it would be desirable to provide an angle of attack indicating system which is instantaneously responsive so that the pilot could maintain the aircraft at the best approach angle of attack and, therefore, a constant glide angle. This would enable the pilot to fly the aircraft closer to the stall angle with confidence especially if it were necessary to land in a relatively short runway distance.